Create slots

Creating slots is a scheduled ("cronjob") function call in the Reservation service.

The root use case must be the slot creation because without it we don't have much of anything. It's fair to call this an "under the hood" type of use case.

For it to do something meaningful, Slots need to be created into the main DynamoDB table so these can be reproduced by the Display service.

This particular solution is trivial, as we only need to allow it to be called as a scheduled function. In our case we call it at 05:00 GMT every Monday through Friday:

code/Reservation/Reservation/serverless.yml

CreateSlots:
  handler: src/infrastructure/adapters/web/CreateSlots.handler
  description: Create new slots
  events:
    # You can activate this to allow for HTTP-based calls
    #- http:
    # method: GET
    # path: /CreateSlots
    - schedule: cron(0 5 ? * MON-FRI *)

The use case itself doesn't do much other than defer to the ReservationService to create the slots.

code/Reservation/Reservation/src/application/usecases/CreateSlotsUseCase.ts

export async function CreateSlotsUseCase(
  dependencies: Dependencies
): Promise<string[]> {
  const reservationService = new ReservationService(dependencies);

  return await reservationService.makeDailySlots();
}

Updating the Display projection

What about that Display service? That one is independently subscribing to relevant Domain Events, so it will update the projection to match what is happening in the leading data source, or Aggregate, which is...Reservation!

code/Reservation/Display/serverless.yml

UpdateSlot:
  handler: src/infrastructure/adapters/web/UpdateSlot.handler
  description: Update a room slot projection
  events:
    # Can be activated if you need to do HTTP-based calls or testing
    #- http:
    # method: POST
    # path: /update
    - eventBridge:
  eventBus: ${self:custom.aws.domainBusArn}
  pattern:
  detail-type:
    # User events
    - "Created"
    - "Cancelled"
    - "Reserved"
    - "CheckedIn"
    - "CheckedOut"
    - "Unattended"
    # System events
    - "Closed"
  source:
    - prefix: ""
  iamRoleStatements:
    - Effect: "Allow"
  Action:
    - dynamodb:PutItem
  Resource: ${self:custom.aws.databaseArn}

As we will see many times, also here the use case is basic. Because our Repository uses an upsert pattern (update in place) it doesn't matter if the item already exists or not; so it's idempotent which is a good thing.

code/Reservation/Display/src/application/usecases/UpdateSlotUseCase.ts

export async function UpdateSlotUseCase(
  dependencies: Dependencies,
  slot: Slot
) {
  const { repository } = dependencies;
  await repository.add(slot);
}